This invention pertains to a moisture detecting probe particularly adapted for use as a part of a snow melter control system and, for purposes of exemplification, will be described in this connection. There has long been a need for means for detecting the presence of moisture, in the form of rain, sleet or snow, on surfaces which present a hazard under freezing conditions. For example, the occurrence of sleet or snow on bridges, parking lots, airport runways, sidewalks, railroad track switches, and so forth always presents a dangerous condition. To alleviate this condition, common practice has been to install heating cables. When frozen moisture conditions exist, the heating cables can be switched on, raising the temperature to melt the accumulated moisture and prevent unsafe conditions from arising.
A problem has existed in providing the necessary controls for automatically switching on and off the heating cables. Two conditions must exist before heating cables need to be switched on. That is, there must be the presence of water in some form, whether sleet, rain or snow, combined with freezing temperatures. The temperature may be detected in the surface itself or in the air above the surface. While the detection of temperature is relatively simple, a difficult problem has existed in detecting the presence of water. Prior probes become inoperative when debris accumulates thereon. Under severe snowing and snow drifting conditions, they also become inoperative because the ice, snow or sleet accumulates thereon and builds up like an igloo to the extent that prior probes are unable to melt the accumulating precipitation and are unable to detect new precipitation. In effect, in severe weather, these prior probes are effectively removed from being exposed to continuing or new precipitation by a build-up of snow or ice and are thus rendered ineffective for detecting such precipitation. Also, prior probes can not withstand heavy industrial uses.
It is the principal object of this invention to provide an improved moisture probe adaptable, as an example of its application, for use in a system to switch on and off heating cables or other similar devices.
A further object of the invention is to provide a moisture sensing probe constructed to forestall accumulation of debris thereon, yet if any debris should accumulate on the probe, its structure permits continued operation.
A further object of the invention is to provide a moisture sensing probe having heating means to heat the entire length of the electrodes under freezing conditions to assure continuous operation of the probe.
A further object of the invention is to provide a heating probe of very sturdy construction to withstand heavy industrial use.
A further object of the invention is to provide a moisture sensing probe of the above type that is simple in construction, inexpensive to manufacture, and highly effective in operation.